Stents are generally tubular devices used to prop open a segment of blood vessel or other anatomical lumen. They are useful in the treatment of atherosclerotic stenosis in blood vessels, maintaining blood perfusion to downstream tissue after opening of a flow restriction.
Various different types of stent designs have been developed for treating diseases of the blood vessels and other tubular structures inside the body. The currently available stents can be classified into two broad categories: balloon-expandable and self-expanding.
A balloon-expandable stent, as described in U.S. Pat. No. 4,776,337, is crimped down onto a folded balloon on the end of a balloon dilatation catheter. When the stent has been properly positioned within the vessel lumen, the balloon is inflated to an appropriate pressure, opening the stent to the desired diameter. The balloon is deflated and the stent remains in its expanded state, due to the plastic deformation that was imparted to its structural elements during expansion.
A balloon-expandable stent has many attractive attributes. Its diameter and outward force to the vessel wall can be adjusted by the inflation pressure of the balloon. After deployment, the stent is a semi-rigid structure that can conform to some extent longitudinally, but maintains a rigid scaffolding that prevents vessel collapse in the radial direction. However one disadvantage to balloon-expandable stents is that there is typically some component of elastic recoil after expansion as long as the mechanism for change between the crimped state to the expanded state is through deformation of the structural elements. This usually means that there is a reduction in diameter after the balloon is deflated. The degree of reduction in diameter is related to the material selection, structural design, and degree of inward force from the vessel wall. These factors vary from stent to stent and situation to situation, presenting a challenge for the practitioner to achieve the desired outcome in repeatable manner.
Traditional balloon-expandable stents change configuration from the crimped to the expanded state through the opening of the angle between radial support members during balloon expansion. In the process, cold work is imparted at the intersections of the structural radial expansion units. Once the expansion is complete, the crystalline structure of the bulk material at the intersections remains in the expanded configuration, minus the minimal elastic recovery.
Achieving this described effect is entirely dependent on the bulk properties of the stent material. For this reason, the material selection is limited to a material that plastically deforms at relatively low strain levels, with a minimal degree of elastic recovery. Materials that fit this description are typically metals. These material requirements are directly in conflict with other secondary desirable attributes of a stent, such as flexibility, biodegradability, and the ability to serve as a platform for drug delivery.
For these reasons, metal is a sub-optimal material selection for these secondary performance categories. However, expandability and radial strength are both primary requirements of a stent, and so metals have been the most viable material option in present day balloon-expandable stents.
Balloon-expandable and self-expanding stents are known that employ ratcheting or latching means for expansion and retaining the expanded configuration. One purported benefit of stent designs that contain latching elements is the capability for more precise lumen sizing. In the balloon-expandable latching stent designs, a latch allows radial expansion but limits post deployment reduction in diameter. In the self-expanding case, a latch can be employed to prevent over-expansion. The latch also provides an upper limit to the chronic outward force on the vessel.
Perhaps the most important benefit of a latching stent design is that the expansion mechanism is not entirely dependent on the bulk deformation of the stent material. This benefit makes possible the use of non-metallic materials in the construction of a latching stent and potentially enables the use of a material that would be better suited to optimize the more secondary performance attributes, such as flexibility, biodegradability, and drug delivery.
However, an important distinction between this prior art and the present invention is that the previously described latching mechanisms are on the same order of scale as the other stent design elements. Furthermore, no specialized micro-fabrication method is specified in order to create the latching elements. There are several undesirable characteristics that result from these important differences.
In practice, one of the general drawbacks to stent-latching mechanisms has been that the latching mechanisms themselves add an additional element that provides additional bulk to the device. For this reason the inclusion of a conventional latch presents reduced flexibility and a larger undeployed profile, i.e., diameter. These characteristics are important because they relate to the ability for the stent to be able to reach a desired delivery site. The flexibility of the stent is a major factor in how well the stent is able to navigate turns in the vessel, and the diameter of the stent determines the minimum cross-sectional restrictions in the vessel that can be traversed by the stent prior to reaching the delivery site. Additionally a larger profile stent requires that larger accessory devices be used to introduce the device. This means that the puncture site to the vessel for introducing the stent must be larger as well, leading to longer post-procedure patient recovery times.
Another drawback to standard stent-latching mechanisms has been that the sizing increment is not continuous. Thus if a desired stent diameter falls between two latch states, the stent must be adjusted to a size which is either too large or too small for the intended application. The sizing increment that is available to the user is typically a function of the size and spacing between latching mechanisms. So the expanded stent diameter increment of adjustability is restricted, when compared to non-latching, balloon-expandable stents. This effect becomes more significant as the size of the target vessel becomes smaller, and so the use of the previously proposed ratcheting stents are practical only with larger, non-coronary vessels.
Thus there is a need for a stent with a latching mechanism which does not appreciably add to the size of the stent or reduce the interior diameter of the stent.
There is a further need for a stent with a latching mechanism which provides virtually continuous adjustment, that is, the increment between adjacent latch states is minimal.